The present invention relates to a composite light-emitting device including a light-emitting element such as light-emitting diode or laser diode, which is implemented as multiple semiconductor layers stacked on a transparent substrate, and a resin member. The resin member may contain a photofluorescent compound that shifts the wavelength of the radiation emitted from the light-emitting element or a filtering compound that partially absorbs the radiation. The present invention also relates to a semiconductor light-emitting unit including the composite light-emitting device and to a method for fabricating the same.
A technique of shifting the wavelength of radiation emitted from a light-emitting element using some phosphor has been well known in the art. For example, according to a technique, the inner wall of a glass neon tube is coated with a phosphor, thereby changing orange light into green one. As an alternative, a photofluorescent compound is added into a molding resin compound for a gallium arsenide (GaAs) light-emitting diode (LED) to convert red emission into green one. Recently, a white LED lamp, which emits white light by coating a blue-light-emitting diode of a Group III nitride semiconductor like gallium nitride (GaN) with a photofluorescent compound, was put on the market. In this specification, such an LED will be simply referred to as a xe2x80x9cGaN LEDxe2x80x9d.
Hereinafter, a prior art white LED lamp will be described with reference to the accompanying drawings.
FIG. 14 illustrates a cross-sectional structure of a conventional white LED lamp. As shown in FIG. 14, the lamp includes: a first leadframe 100A, which is provided with a reflective cup 100a at the end; and a second leadframe 100B, the end of which is spaced apart from that of the reflective cup 100a. A GaN LED 110 is bonded onto the bottom of the reflective cup 100a with an insulating adhesive 101. One of the electrodes of the GaN LED 110 is connected to the first leadframe 100A with a first wire 102A, while the other electrode thereof is connected to the second leadframe 100B with a second wire 102B. The reflective cup 100a is filled in with a wavelength-shifting resin medium 104, which contains a photofluorescent compound that shifts the wavelength of the radiation emitted from the GaN LED 110, so as to cover the GaN LED 110. The upper ends of the first and second leadframes 100A and 100B, as well as the reflective cup 100a, are molded together within a spherical resin encapsulant 105 such as transparent epoxy resin to form the white LED lamp.
Although not shown in any drawing, a chip LED may also be formed without using the reflective cup 100a or the spherical resin encapsulant 105. In the chip LED, the GaN LED 110 is mounted onto a concave receptacle within a casing and then the gap between the LED 110 and the receptacle is filled in with a resin encapsulant containing a photofluorescent compound to secure them together.
Next, a detailed construction of the known GaN LED 110 will be described.
FIGS. 15(a) and 15(b) illustrate the GaN LED for use in the conventional white LED lamp: FIG. 15(a) illustrates a planar layout thereof; and FIG. 15(b) illustrates a cross-sectional structure thereof taken along the line XVbxe2x80x94XVb in FIG. 15(a). As shown in FIG. 15(b), the GaN LED 110 includes n-type GaN contact layer 112, quantum well structure and p-type GaN contact layer 116, which are stacked in this order over a sapphire substrate 111. The quantum well structure is formed on part of the upper surface of the n-type contact layer 112 and includes n-type AlGaN first barrier layer 113, InGaN single quantum well (SQW) layer 114 and p-type AlGaN second barrier layer 115.
Also, as shown in FIG. 15(a), an n-side electrode 117 is formed on the exposed part of the upper surface of the n-type contact layer 112. A current-diffusing transparent electrode 118 is formed on the p-type contact layer 116. And a p-side electrode 119 is formed on the transparent electrode 118 to be located farthest from the n-side electrode 117.
Since the conventional GaN LED 110 is formed on the insulating sapphire substrate 111, both the n- and p-side electrodes 117 and 119 are provided on the same side of the substrate 111 as that including the LED thereon.
The conventional white LED lamp shown in FIG. 14 or the chip LED (not shown) covers the GaN LED 110 by filling in the reflective cup 100a or the receptacle of the casing with the wavelength-shifting resin medium 104 containing the photofluorescent compound 103. Thus, the prior art construction is not applicable to a light-emitting unit including no such reflective cup 100a or receptacle.
Also, if the reflective cup 100a or receptacle should be filled in with the wavelength-shifting resin medium 104, it is difficult to precisely control the amount of the resin medium to be filled in or the variation in concentration of the photofluorescent compound 103. Thus, the chromaticity changes significantly. As a result, the yield of good light-emitting units with a desired chromaticity decreases.
Furthermore, the GaN LED 110 included in the white LED lamp or chip LED is the same as that included in a blue LED lamp. The blue-light-emitting diode is poorly resistant to static electricity due to the physical constants (like the relative dielectric constant ∈) of the constituent materials thereof or the structure thereof.
A first object of the present invention is getting a composite light-emitting device always covered with a wavelength-shifting resin medium irrespective the shapes of leadframes or casings.
A second object of the present invention is improving the resistance of a composite light-emitting device or semiconductor light-emitting unit to an overvoltage caused by static electricity.
A third object of the present invention is making the chromaticity of the emission finely adjustable while at the same time suppressing the variation in chromaticity.
To achieve the first object, an inventive composite light-emitting device includes a light-emitting element and a submount member. The light-emitting element with an active region defined on a transparent substrate is mounted facedown on the submount member with the active region of the light-emitting element facing the principal surface of the submount member. The submount member is electrically connected to the light-emitting element. And the light-emitting element is covered with a wavelength-shifting resin medium on the principal surface of the submount member.
To accomplish the second object, the submount member is implemented as an overvoltage protector.
To attain the third object, the light-emitting face of the substrate for the light-emitting element on the opposite side to its circuitry side and/or the outer surface of the wavelength-shifting resin medium above the light-emitting face are/is made parallel to the back surface of the submount member.
Specifically, a composite light-emitting device according to the present invention includes a light-emitting element including a transparent substrate and a multilayer structure formed on the substrate. The multilayer structure includes first and second semiconductor layers of first and second conductivity types, respectively. The device further includes a submount member for mounting the light-emitting element thereon. The principal surface of the submount member faces the multilayer structure. The submount member is electrically connected to the light-emitting element. The device further includes a wavelength-shifting resin member, which is provided on the principal surface of the submount member to cover the light-emitting element. The wavelength-shifting resin member contains a photofluorescent or filtering compound. The photofluorescent compound shifts the wavelength of radiation that has been emitted from the light-emitting element. The filtering compound partially absorbs the radiation.
In the composite light-emitting device according to the present invention, the multilayer structure of the light-emitting element, which functions as active region of the element, is flip-chip bonded to the principal surface of the submount member. And the radiation emitted is allowed to pass through the backside of the substrate for the light-emitting element. Accordingly, the submount member, on which the light-emitting element is mounted, supports the wavelength-shifting resin member thereon, or acts as a receptacle for the resin member. Thus, the light-emitting element can be covered with the wavelength-shifting resin member irrespective of the shape of a leadframe or a mount of a casing.
In one embodiment of the present invention, the principal surface of the submount member is preferably greater in area than that of the substrate for the light-emitting element and is preferably rectangular with a side of about 0.25 mm or more.
In another embodiment, the light-emitting element preferably includes: a first electrode formed on the multilayer structure and electrically connected to the first semiconductor layer; and a second electrode electrically connected to the second semiconductor layer. The submount member is preferably an overvoltage protector including first and second counter electrodes, which are formed on the principal surface thereof so as to face the first and second electrodes of the light-emitting element, respectively. When a voltage, which is lower than a dielectric breakdown voltage but exceeds a predetermined voltage, is applied between the first and second electrodes of the light-emitting element, a current preferably flows between the first and second counter electrodes.
In such an embodiment, even if a voltage equal to or higher than a dielectric breakdown voltage is applied between the first and second electrodes of the light-emitting element due to static electricity, for example, a bypass current flows between the two electrodes of the overvoltage protector. As a result, the light-emitting element can be protected without causing any dielectric breakdown.
In this particular embodiment, the first and second electrodes of the light-emitting element are preferably n- and p-side electrodes, respectively, and the overvoltage protector is preferably a diode using the first and second counter electrodes as anode and cathode, respectively.
More specifically, a forward operating voltage of the diode is preferably lower than a reverse dielectric breakdown voltage of the light-emitting element. And a reverse breakdown voltage of the diode is preferably higher than an operating voltage of the light-emitting element but lower than a forward dielectric breakdown voltage of the light-emitting element.
In an alternate embodiment, the first and second electrodes are preferably connected electrically to the first and second counter electrodes, respectively, with microbumps interposed therebetween.
Specifically, the microbumps are preferably fused and bonded together with the associated electrodes facing the bumps. The overvoltage protector preferably includes a backside electrode on another surface thereof opposite to the principal surface. One of the first and second counter electrodes preferably includes a bonding pad to be electrically connected to an external component. And the polarity of the backside electrode is preferably opposite to that of the first or second counter electrode that includes the bonding pad.
More particularly, the first and second semiconductor layers of the light-emitting element are preferably made of Group III nitride compound semiconductors, and the overvoltage protector is preferably a lateral diode made of silicon. P- and n-type semiconductor regions are defined in an upper part thereof closer to the principal surface.
In another embodiment of the present invention, the light-emitting element may include: a first electrode formed on the multilayer structure and electrically connected to the first semiconductor layer; and a second electrode electrically connected to the second semiconductor layer. The submount member may be an auxiliary member made of a conductor. The submount member may include: a first counter electrode, which is formed on the principal surface thereof so as to face the first electrode of the light-emitting element and is electrically isolated from the conductor; and a second counter electrode, which is formed on the principal surface thereof so as to face the second electrode of the light-emitting element and is electrically continuous with the conductor.
In this particular embodiment, the first and second electrodes are preferably connected electrically to the first and second counter electrodes, respectively, with microbumps interposed therebetween.
Specifically, the microbumps are preferably fused and bonded together with the associated electrodes facing the bumps. The auxiliary member preferably includes a backside electrode on another surface thereof opposite to the principal surface. One of the first and second counter electrodes preferably includes a bonding pad to be electrically connected to an external component. And the backside electrode is preferably continuous electrically with the first or second counter electrode that includes no bonding pads.
More particularly, the first and second semiconductor layers of the light-emitting element are preferably made of Group III nitride compound semiconductors, and the auxiliary member is preferably made of conductive silicon.
In still another embodiment, the wavelength-shifting resin member is preferably made of a transparent resin containing the photofluorescent compound at about 50 to about 90 percent by weight.
In this particular embodiment, a light-emitting surface of the substrate for the light-emitting element on the opposite side to another surface thereof on which the multilayer structure is formed and/or an outer surface of part of the wavelength-shifting resin member above the light-emitting surface are/is preferably substantially parallel to a surface of the submount member on which the backside electrode is formed. This is because the variation in chromaticity of the emitted radiation, which usually greatly depends on the thickness of that part of the wavelength-shifting resin member above the light-emitting surface of the light-emitting element, can be suppressed by doing so.
Specifically, the thickness of the part of the wavelength-shifting resin member above the light-emitting surface is preferably in a range from approximately 20 xcexcm to approximately 100 xcexcm, both inclusive.
Alternatively, the thickness of a part of the wavelength-shifting resin member covering the light-emitting surface and sides of the light-emitting element is preferably in the range from approximately 20 xcexcm to approximately 110 xcexcm, both inclusive.
A semiconductor light-emitting unit according to the present invention includes: a composite light-emitting device including a light-emitting element and a submount member for mounting the light-emitting element thereon; a leadframe or wiring board including a mount for supporting a surface of the submount member on the opposite side to the principal surface thereof on which the light-emitting element is mounted; and a transparent resin encapsulant covering the mount as well as the composite light-emitting device. The light-emitting element includes a transparent substrate and a multilayer structure formed on the substrate. The multilayer structure includes first and second semiconductor layers of first and second conductivity types, respectively. The principal surface of the submount member faces the multilayer structure. The submount member is electrically connected to the light-emitting element. And the light-emitting element is covered with a wavelength-shifting resin member, which is provided on the principal surface of the submount member and contains a photofluorescent or filtering compound. The photofluorescent compound shifts the wavelength of radiation that has been emitted from the light-emitting element. The filtering compound partially absorbs the radiation.
An inventive semiconductor light-emitting unit can be formed easily to include the composite light-emitting device of the present invention.
In one embodiment of the present invention, the submount member preferably includes a bonding pad, which is formed on the principal surface thereof and electrically connected to the light-emitting element. The submount member preferably further includes a backside electrode, which is formed on another surface thereof opposite to the principal surface on which the light-emitting element is mounted. The backside electrode and the mount are preferably bonded together with a conductive paste. And the bonding pad is preferably connected electrically to a member other than the mount via a wire.
A first exemplary method for fabricating a semiconductor light-emitting unit according to the present invention includes the step of making a light-emitting element by forming a multilayer structure on a transparent substrate and forming electrodes on the multilayer structure. The multilayer structure includes first and second semiconductor layers of first and second conductivity types, respectively. The method further includes the step of making a submount member including counter electrodes on the principal surface thereof. The counter electrodes face the electrodes of the light-emitting element. The method further includes the steps of: forming microbumps on the electrodes or on the counter electrodes; mounting the light-emitting element facedown on the principal surface of the submount member such that the multilayer structure faces the principal surface and that the electrodes of the light-emitting element are electrically connected to the counter electrodes via the microbumps; and coating the light-emitting element with a wavelength-shifting resin medium that will be supported on the principal surface of the submount member when cured. The wavelength-shifting resin medium contains a photofluorescent or filtering compound. The photofluorescent compound shifts the wavelength of radiation that has been emitted from the light-emitting element. The filtering compound absorbs the radiation partially.
According to the first method of the present invention, the inventive semiconductor light-emitting unit can be fabricated just as intended.
A second exemplary method for fabricating a semiconductor light-emitting unit according to the present invention includes the step of a) making multiple light-emitting elements by forming a multilayer structure on each of a plurality of transparent substrates and forming electrodes on the multilayer structure. The multilayer structure includes first and second semiconductor layers of first and second conductivity types, respectively. The method further includes the step of b) making multiple submount members on a wafer. Each said submount member includes: a bonding pad on the principal surface thereof; counter electrodes that face the electrodes of associated one of the light-emitting elements; and a backside electrode on another surface thereof opposite to the principal surface. The method further includes the steps of: c) forming microbumps on the electrodes or on the counter electrodes by a stud bump forming or plating technique; d) bringing the electrodes of each said light-emitting element into contact with the counter electrodes of associated one of the submount members via the microbumps and fusing the microbumps by applying ultrasonic waves or heat thereto such that the microbumps and the associated electrodes are bonded together, thereby electrically connecting the light-emitting elements to the associated submount members and bonding the light-emitting elements onto the respective principal surfaces of the submount members; and e) coating each said light-emitting element with a wavelength-shifting resin medium that will be supported on the principal surface of the associated submount member and then curing the wavelength-shifting resin medium, thereby obtaining a plurality of composite light-emitting devices, each including one of the light-emitting elements and associated one of the submount members. The wavelength-shifting resin medium contains a photofluorescent or filtering compound. The photofluorescent compound shifts the wavelength of radiation that has been emitted from the light-emitting element and the filtering compound absorbs the radiation partially. The method further includes the steps of: f) dicing the wafer, on which the composite light-emitting devices have been formed, into multiple chips; g) securing each said composite light-emitting device, which is included on one of the chips, onto a mount of a leadframe or a wiring board such that the backside electrode of the submount member thereof is bonded to the mount via a conductive paste; and h) connecting the bonding pad of each said submount member to the associated leadframe or wiring board with a wire.
According to the second method of the present invention, the submount members, which will support the wavelength-shifting resin member thereon, are handled before these members are cut out from the wafer. Thus, the wavelength-shifting resin medium can be supplied not just by using a dispenser but by some patterning technique for a wafer, e.g., screen printing. Accordingly, the resin medium can be shaped accurately and efficiently.
In one embodiment of the present invention, the second method may further include, between the steps d) and e), the step of polishing a light-emitting surface of the substrate for each said light-emitting element on the opposite side to another surface thereof on which the multilayer structure is formed such that the light-emitting surface becomes substantially parallel to a surface of the associated submount member on which the backside electrode is formed.
In an alternate embodiment, the second method may further include, between the steps e) and f), the step of polishing an outer surface of part of the wavelength-shifting resin medium above each said light-emitting element such that the outer surface becomes substantially parallel to the surface of the associated submount member on which the backside electrode is formed.